This invention relates to an optical head apparatus for carrying out recording and reproducing for an optical recording medium, and in particular, to an optical head apparatus capable of detecting both of a tracking error signal by a differential phase method and a tracking error-signal by a push-pull method.
For an optical recording medium of a read only type, such as, DVD-ROM, a differential phase method is generally used as a method of detecting a tracking error signal.
On the other hand, for an optical recording medium of a rewritable type, such as, DVD-RAM, a push-pull method is generally used as a method of detecting a tracking error signal.
Accordingly, in order to deal with both of an optical recording medium of the read only type and an optical recording medium of the rewritable type by a single optical head apparatus, both of a tracking error signal by the differential phase method and a tracking error signal by the push-pull method are required to be detected.
Further, as a method of detecting a focusing error signal, there are generally used a Foucault method (or double knife edge method), an astigmatism method and a spot size method.
In this case, the Foucault method is featured in that noise of the focusing error signal in traversing tracks is smaller than those of the astigmatism method and the spot size method.
Japanese Unexamined Patent Publication (JP-A) No. 143878/1998 and Japanese Unexamined Patent Publication (JP-A) No. 143883/1998 disclose an optical head apparatus capable of detecting both of a tracking error signal by the differential phase method and a tracking error signal by the push-pull method, and also capable of detecting a focusing error signal by the Foucault method.
FIG. 1 shows a structure of a first conventional optical head apparatus disclosed in Japanese Unexamined Patent Publication No. 143878/1998.
Emitted light from a semiconductor laser 1 is formed into a parallel ray by a collimator lens 2, is incident on a polarization beam splitter 3 as p-polarized light, transmits therethrough substantially by 100%, is converted from linearly polarized light into circularly polarized light by a quarter-wave plate 4, and is focused onto a disk 6 by an object lens 5.
Reflected light from the disk 6 transmits through the object lens 5 in a reverse direction, is converted from circularly polarized light into linearly polarized light by the quarter-wave plate 4, is incident on the polarization beam splitter 3 as s-polarized light, reflected thereby substantially by 100%, is diffracted by a hologram optical element 158, transmits through a lens 8 and is received by an optical detector 159.
FIG. 2 is a plane view of the hologram optical element 158. The hologram optical element 158 is divided into four of a region 160 through a region 163 by two dividing lines respectively in parallel with a radial direction and a tangential direction of the disk 6.
FIG. 3 shows a pattern of the optical detector 159 and light spots on the optical detector 159.
The light detector 159 has a light receiving portion 164 through a light receiving portion 171. With this structure, +1st-order diffracted light from the region 160 of the hologram optical element 158 forms a light spot 173 on a boundary line between the light receiving portion 164 and the light receiving portion 165. −1st-order diffracted light therefrom forms a light spot 178 on the light receiving portion 170.
+1st-order diffracted light from the region 161 of the hologram optical element 158 forms a light spot 172 outside of the light receiving portions while −1st-order diffracted light therefrom forms a light spot 179 on the light receiving portion 171.
+1st-order diffracted light from the region 162 of the hologram optical element 158 forms a light spot 174 on a boundary between the light receiving portion 166 and the light receiving portion 167 while −1st-order diffracted light therefrom forms a light spot 177 on the light receiving portion 169.
+1st-order diffracted light from the region 163 of the hologram optical element 158 forms a light spot 175 outside of the light receiving portions while −1st-order diffracted light therefrom forms a light spot 176 on the light receiving portion 168.
When outputs from the light receiving portion 164 through the light receiving portion 171 are respectively designated by notations V164 through V171, the focusing error signal by the Foucault method is obtained by calculation of (V164+V167)−(V165+V166).
The tracking error signal by the differential phase method is obtained from a phase difference between V168+V170 and V169+V171.
Further, the tracking error signal by the push-pull method is obtained from calculation of (V168+V171)−(V169+V170).
Moreover, a data signal recorded on the disk 6 is obtained from calculation of V168+V169+V170+V171 or V164+V165+V166+V167+V168+V169+V170+V171.
FIG. 4 shows a structure of a module 180 which is a principal portion of the second conventional optical head disclosed in Japanese Unexamined Patent Publication No. 143883/1998.
A semiconductor laser 181 and an optical detector 182 are installed inside the module 180, and a hologram optical element 183 is arranged at a window portion of the module 180.
Emitted light from the semiconductor laser 181 partially transmits through the hologram optical element 183, and progresses toward a disk. Reflected light from the disk is partially diffracted by the hologram optical element 183, and is received by the optical detector 182.
FIG. 5 is a plane view of the hologram optical element 183. The hologram optical element 183 is divided into four of a region 184 through a region 187 by two dividing lines respectively in parallel with a radial direction and a tangential direction of the disk.
FIG. 6 shows a pattern of the optical detector 182 and light spots on the optical detector 182.
The optical detector 182 is provided with a light receiving portion 188 through a light receiving portion 193. −1st-order diffracted light from the region 184 of the hologram optical element 183 forms a light spot 195 on a boundary line between the light receiving portion 189 and the light receiving portion 190.
+1st-order diffracted light from the region 185 of the hologram optical element 183 forms a light spot 194 on the light receiving portion 188.
+1st-order diffracted light from the region 186 of the hologram optical element 183 forms a light spot 197 on the light receiving portion 193.
+1st-order diffracted light from the region 187 of the hologram optical element 183 forms a light spot 196 on a boundary line between the light receiving portion 191 and the light receiving portion 192.
When outputs from the light receiving portion 188 through the light receiving portion 193 are designated by notations V188 through V193, the focusing error signal by the Foucault method is obtained by calculation of (V189+V192)−(V190+V191). The tracking error signal by the differential phase method is obtained by a phase difference between V189+V190+V191+V192 and V188+V193.
The tracking error signal by the push-pull method is obtained from calculation of (V189+V190+V193)−(V188+V191+V192). Further, a data signal recorded on the disk is obtained from calculation of V188+V189+V190+V191+V192+V193.
FIG. 7 is a sectional view of the hologram optical element 183. The hologram optical element 183 is constituted so that a dielectric film 198 is formed on a glass substrate 14.
With such a structure, emitted light from the semiconductor laser 181 is incident as incident light 199 on the hologram optical element 183, transmits therethrough as transmitted light 200, and progresses toward the disk.
Reflected light from the disk is incident as incident light 201 onto the hologram optical element 183, is diffracted as +1st-order diffracted light 202, and is received by the optical detector 182.
By forming a sectional shape of the dielectric film 198 in a sawtooth-like shape, the diffraction efficiency of the +1st-order diffracted light is enhanced, and almost no −1st-order diffracted light is generated.
In the first conventional optical head apparatus, the data signal recorded on the disk 6 is obtained by the calculation of V168+V169+V170+V171 or V164+V165+V166+V167+V168+V169+V170+V171.
In the latter case, the light spot 173 formed on the boundary between the light receiving portion 164 and the light receiving portion 165 and the light spot 174 formed on the boundary line between the light receiving portion 166 and the light receiving portion 167, are used for detecting the data signal.
However, the frequency characteristic as an optical detector on the boundary line is lower than that on the light receiving portion. Therefore, the optical spot formed on the boundary line does not substantially contribute to detecting the data signal which is a high frequency signal.
Hence, consider only a case in that the data signal recorded on the disk 6 is obtained from calculation of V168+V169+V170+V171.
High S/N is requested to the data signal recorded in the disk 6 and the tracking error signal by the differential phase method because both of them are high frequency signals.
In order to achieve high S/N, a ratio A of an optical amount used in detecting these signals to an optical amount of the reflected light from the disk 6 is needed to be as large as possible.
A sectional shape of the hologram optical element 158 is rectangular. Therefore, the diffraction efficiency of the +1st-order diffracted light and the diffraction efficiency of the −1st-order diffracted light are equal to each other.
In this case, maximum values of the diffraction efficiencies of the ±1st-order diffracted light are about 40.5%, respectively. Namely, the maximum value of the above-mentioned A is equal to 0.405. The value is not necessarily regarded as sufficiently large.
In the second conventional optical head apparatus, the data signal recorded on the disk is obtained by the calculation of V188+V189+V190+V191+V192+V193. In this case, the optical spot 195 formed on the boundary line of the light receiving portion 189 and the light receiving portion 190 and the optical spot 196 formed on the boundary line of the light receiving portion 191 and the light receiving portion 192, are used for detecting the data signal.
However, the frequency characteristic as an optical detector on the boundary line is lower than that on the light receiving portion. Accordingly, the optical spot formed on the boundary line does not substantially contribute to detecting the data signal that is a high frequency signal.
That is, this is equivalent to detecting the data signal by only using the optical spot 194 and the optical spot 197 in correspondence with a half of the section for the reflected light from the disk. Consequently, the resolution of the data signal and crosstalk between contiguous tracks are deteriorated, and the data signal cannot be correctly detected.
Further, the focusing error signal is detected by only using the light spot 196 and the light spot 196 corresponding to a half in the section of the reflected light from the disk. In consequence, noise of the focusing error signal in traversing tracks becomes large, and the focusing error signal cannot be correctly detected.
There is conceivable a structure in which in place of the hologram optical element 158 in the first conventional optical head apparatus, the hologram optical element 183 in the second conventional optical head apparatus is used, the focusing error signal is detected from the −1st-order diffracted light and the tracking error signal by the differential phase method and the tracking error signal by the push-pull method and the data signal recorded on the disk 6 are detected from the +1st-order diffracted light.
However, in this case, the diffraction efficiency of the +1st-order diffracted light is high. Therefore, the above-mentioned value A can be increased. But, almost no −1st-order diffracted light is generated. In consequence, the focusing error signal cannot be actually detected.